Due to recent trends toward more compact and lighter portable electronic equipment, there has been a growing need to develop a high performance and large capacity battery to power this portable electronic equipment. A lithium secondary battery, having an average discharge potential of 3.7 V (i.e., a battery having substantially a 4 V average discharge potential) is considered to be an essential element in the digital generation since it is an indispensable energy source for portable digital devices such as cellular telephones, notebook computers, and camcorders. (i.e., the “3C” devices).
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7V, which is higher than alkali batteries, Ni-MH batteries, Ni—Cd batteries and the like. An electrolyte that is electrochemically stable in the charge and discharge voltage range of 0 to 4.2V is required in order to generate such a high driving voltage. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate is used as an electrolyte. However, such an electrolyte has significantly lower ion conductivity than an aqueous electrolyte that is used in a Ni-MH battery or a Ni—Cd battery, thereby resulting in the deterioration of battery characteristics during charging and discharging at a high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, and the like, thereby forming a thin film on the surface of the negative electrode. This film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents disintegration of the structure of the carbon negative electrode, which is caused by co-intercalation of organic solvents having a high molecular weight along with solvated lithium ions into the carbon negative electrode.
Once the organic SEI film is formed, lithium ions do not react again with the carbon electrode or other materials, such that an amount of lithium ions is maintained. That is, carbon of the negative electrode reacts with an electrolyte during the initial charging, thus forming a passivation layer such as an organic SEI film on the surface of the negative electrode such that the electrolyte solution no longer decomposes, and stable charging and discharging are maintained (J. Power Sources, 51(1994), 79-104). For these reasons, in the lithium secondary battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
However, gases are generated inside the battery due to decomposition of a carbonate-based organic solvent during the organic SEI film-forming reaction (J. Power Sources, 72(1998), 66-70). These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, and the like depending on the type of non-aqueous organic solvent and negative active material used. The thickness of the battery increases during charging due to the generation of gases inside the battery.
The passivation layer slowly disintegrates by electrochemical energy and heat energy, which increases with the passage of time when the battery is stored at a high temperature after it is charged. As a result, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolyte occurs continuously. The internal pressure of the battery increases with this generation of gases, inducing the deformation of prismatic batteries or pouch batteries. As a result, regional differences in the cohesion among electrodes inside the electrode assembly (positive and negative electrodes, and separator) of the battery occur, thereby deteriorating the performance and safety of the battery and making it difficult to mount the lithium secondary battery set into electronic equipment.
In order to improve low temperature characteristics, a lithium secondary battery having a liquid electrolyte uses an organic solvent with a low boiling point which induces swelling of a prismatic or pouch battery during high temperature storage. As a result, the reliability and safety of the battery deteriorate at a high temperature.
Accordingly, extensive research into a liquid electrolyte with a high boiling point is needed. An example of an electrolyte with a high boiling point is an ester solvent such as gamma butyrolactone. When using 30 to 70% of an ester solvent, cycle life characteristics significantly deteriorate and therefore it is difficult to use it for batteries. It has been suggested that as an electrolyte with a high boiling point, a mixture of gamma butyrolactone/ethylene carbonate (7/3) can be used, and a boron-coated mesocarbon fiber (MCF) as a negative active material can be used to reduce swelling at a high temperature and improve cycle life characteristics (Journal of Electrochemical Society, 149(1) A(9)˜A12(2002)). However, when an uncoated carbonaceous material is used as a negative active material, cycle life characteristics deteriorate even when an electrolyte with a high boiling point is used. Therefore, there is a need to pursue research on electrolytes to improve cycle life as well as swelling inhibition properties at high temperature.